On April 20 last year, the Deepwater Horizon oil rig exploded. Eleven men died, and two days later the rig began spewing oil into the Gulf of Mexico. Five million barrels of oil would gush out before BP was able to cap the well in July. The company would also spray 1.8 million gallons of­­ chemical dispersant to prevent at least some of it from reaching shore. Still, 2.5 million pounds of oil hit Florida's shores. A year later, though, "the data tells us the water is safe, the seafood is safe and the beaches are safe," said Keith Lee Rupp of BP America. Despite that rosy take, efforts to assess the damage from the largest spill in U.S. history are far from complete. Scientists are probing the ocean bottom, and examining the water, the corals, the oysters, the fish, the sand on the beaches. They have found more questions than answers.

In February a University of Georgia scientist named Samantha Joye reported her findings from a trip into the gulf in a submarine: Starfish, dead. Crabs, dead. Worms, dead. Scientists with the University of South Florida have found deformed shells among small bottom-dwelling creatures and an unexplained increase in the amount of sediment coating the ocean floor — so much that it could smother the remaining life there. There are other species at risk — bluefin tuna, for instance, already considered endangered because their stock had decreased by 80 percent since 1970, were spawning during the spill — but no one knows the impact on them yet. Oyster beds are struggling. Meanwhile a total of 12 visibly oiled dolphins — two of them alive, 10 already dead — have washed up along the Gulf Coast. As recently as March, one washed ashore coated in oil that was confirmed to come from Deepwater Horizon. But federal officials say they cannot at this point say oil killed the dolphins. They say the same thing about the scores of newborn and stillborn dolphins that washed up earlier this year. Scientists from the University of British Columbia last month estimated that for every dead whale or dolphin that has washed ashore, there are 50 more that died at sea and were never seen by humans.

Birds and other wildlife

A photo of an oil-coated pelican became the symbol of the spill, while sea turtles were the focus of its boldest rescue effort. By February, federal wildlife officials had counted more than 6,000 birds and more than 1,000 turtles that had been killed by the spill (although a report this month from an environmental group, the Center for Biological Diversity, contended the number of birds is really 10 times higher and turtles six times higher). Biologists are now monitoring the spring nesting numbers to see if there are any immediate changes in the bird population. As for the turtles: More than half the 25,000 eggs that were moved from the oil-coated gulf coast to Kennedy Space Center successfully hatched nestlings that made it into the water safely. Whether they survive or find their way back to lay their own eggs will not be known for years.

Air quality

Last June, scientists in aircraft who were sampling the sky above the spill zone found a 24-mile-wide plume of evaporated oil was polluting the air, they revealed last month. The aerosol compound contained particles that could damage the heart and lungs of anyone who inhaled it, primarily the crews working on the spill. Although the pollution may have affected workers, there are no indications that it reached land or had any effect on residents. The EPA collected more than 1.4 million air samples along the Gulf Coast throughout the Deepwater Horizon crisis, and BP officials point out that none indicated pollution at levels harmful to humans.

The beaches

The sugar-white sands of the Florida Panhandle are once again white, although not quite as pristine as they used to be. Back in June, scientists found that even beaches that had been declared clean had thick layers of oil buried inches beneath the sand. During the summer BP brought in machinery that dug up the sand and shook it through a sieve, straining out the tarballs but mixing together the remaining contaminated and uncontaminated sand. The result: no more buried oil, but the sand is now mixed with tiny bits of oil. It make take years for bacteria to eat the remaining oil particles. Meanwhile mats of weathered oil remain just offshore, but BP announced last week it would dig those out or vacuum them up. BP still has 240 people assigned to monitor Florida's coast for any new oil, with plans to keep checking through the end of this year's hurricane season.

The wetlands

Coastal marshes in Louisiana, many damaged by repeated tides of thick oil during the spill, are still being hit by oil a year later. The marshes are crucial nurseries for marine life, as well as habitat for migratory birds. New shoots have sprung up in some marshes, and volunteers have planted new marsh grass and mangroves, hoping to replace what's lost. But scientists say they won't know for years whether the marshes will rebound.

The gulf

During the spill, USF researchers were one of the groups who found underwater plumes of oil snaking along for miles in the deep gulf. In December, when USF crews returned to the same areas, those oil plumes appeared to be gone, said scientist David Hollander. However, Hollander and his colleagues found a huge increase in the amount of sediment from dead plant and animal material littering the gulf floor, and expressed concerns about whether it would smother the small creatures that inhabit the floor. Samantha Joye of the University of Georgia reported finding strings of bacterial slime that created what she called an "invertebrate graveyard."

What we missed

Florida's best ally turned out to be nature. Winds kept the oil from washing ashore in the Panhandle in the same quantities that hit other states, and kept it from spreading beyond Franklin County. Changes in the loop current kept it from sweeping into the Keys and then along the Atlantic Coast. No major hurricanes struck the gulf to fling lots of oil ashore. As USF oceanographer Robert Weisberg put it, "Florida lucked out."

Human health

From the start of the oil spill there have been unconfirmed reports about people along the Gulf Coast suffering from nausea and other sicknesses. When BP began spraying a chemical dispersant called Corexit on the oil, that prompted even greater fears about health effects. In the past 50 years, in 40 known oil spills around the world, only eight have been studied for human health impacts. They found that cleanup workers exposed to crude oil often suffer acute short-term effects — stinging eyes, rashes, nausea, dizziness, headaches, coughs and other respiratory symptoms — but there have been no studies of the long-term health effects. In February the National Institute of Health launched a study of the health of 55,000 Deepwater Horizon cleanup workers and volunteers in Louisiana, Mississippi, Alabama, and Florida — the largest study in the agency's history. A companion study of health impacts in communities along the coast will follow shortly, said Dr. Dale Sandler, who is leading the study. "I know people believe they're sick from the oil," she said, but it's difficult to prove a cause-and-effect relationship.